feat(tvix/eval): implement if/else expressions

These expressions use simple jumps to skip the correct expression
conditionally in the bytecode by advancing the instruction pointer.

Note that these expressions are already covered by a test behind the
`nix_tests` feature flag, but adding more is probably sensible.

Change-Id: Ibe0eba95d216321c883d3b6b5816e2ab6fe7eef1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6148
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/value/mod.rs

@@ -52,9 +52,9 @@ impl Value {
         }
     }
 
-    pub fn as_bool(self) -> EvalResult<bool> {
+    pub fn as_bool(&self) -> EvalResult<bool> {
         match self {
-            Value::Bool(b) => Ok(b),
+            Value::Bool(b) => Ok(*b),
             other => Err(Error::TypeError {
                 expected: "bool",
                 actual: other.type_of(),